[boost] [lockfree] _ENABLE_ATOMIC_ALIGNMENT_FIX for VS 2015 Update 2

Subject: [boost] [lockfree] _ENABLE_ATOMIC_ALIGNMENT_FIX for VS 2015 Update 2
From: Stephan T. Lavavej (stl_at_[hidden])
Date: 2016-05-09 18:40:24

Hi,

While running Boost.Lockfree's tests, one of our compiler devs encountered the following failure:

freelist_test.cpp
c:\wcfb01\binaries\x86chk\inc\atomic(659): error C2338: You've instantiated std::atomic<T> with sizeof(T) equal to 2/4/8 and alignof(T) < sizeof(T). Before VS 2015 Update 2, this would have misbehaved at runtime. VS 2015 Update 2 was fixed to handle this correctly, but the fix inherently changes layout and breaks binary compatibility. Please define _ENABLE_ATOMIC_ALIGNMENT_FIX to acknowledge that you understand this, and that everything you're linking has been compiled with VS 2015 Update 2 (or later).
c:\wcfb01\binaries\x86chk\inc\atomic(669): note: see reference to class template instantiation 'std::_Atomic_base<_Ty,4>' being compiled
        with
        [
            _Ty=boost::lockfree::detail::tagged_index
        ]
e:\boost\boost\boost\lockfree\detail\freelist.hpp(608): note: see reference to class template instantiation 'std::atomic<boost::lockfree::detail::tagged_index>' being compiled
e:\boost\boost\boost\lockfree\detail\freelist.hpp(609): note: see reference to class template instantiation 'boost::lockfree::detail::fixed_size_freelist<T,NodeStorage>' being compiled

    call "C:\WCFB01\binaries\x86chk\bin\i386\vcvarsall.bat" x86 >nul
cl /Zm800 -nologo @"..\bin.v2\libs\lockfree\test\freelist_test.test\msvc-latest\debug\threading-multi\freelist_test.obj.rsp" 

...failed compile-c-c++ ..\bin.v2\libs\lockfree\test\freelist_test.test\msvc-latest\debug\threading-multi\freelist_test.obj...

with the following type:

class tagged_index
{
public:
    typedef boost::uint16_t tag_t;
    typedef boost::uint16_t index_t;

protected:
    index_t index;
    tag_t tag;
};

This is by design - your tagged_index has align 2, size 4, triggering the need for my atomic alignment fix.  As the static_assert advises, your code needs to define _ENABLE_ATOMIC_ALIGNMENT_FIX in order to pass.

If this has already been fixed in 1.61.0, then that's great (I haven't checked).

Thanks,
STL